On South Beach you do not need to count calories. If you eat the allowed foods in the allowed portions, that will take care of itself. However, some people do count them, and I do on occasion as a sanity check. It's completely up to you. There are lots of different calorie calculators online, but I don't really know how good they are. You can experiment to find a range that lets you lose pounds without feeling hungry. For me, I try to keep my calories in-between 1600 - 1800/day. Any less and I get hungry. I would consider myself moderately active, I exercise most days, but I have a desk job.
Since you just switched from a low-calorie plan, one of the great things you'll notice about SB is that you will find you are eating more high quality calories that fill you up and keep you satisfied and are still able to lose. I think if you focus only on calories, you can eat 1000 calories of junk food and lose for a while, but it's not healthy and you won't be successful in the end.
Since you are just getting started on SB, I would advise you not to worry about the calories, and give the plan a chance to show you it can work. Good luck!
Life's a journey, not a destination.
It's easier to stay on plan than to get back on plan.